FOR a phenomenon with such predictably bad outcomes, a financial boom is strangely seductive. Not a decade after the most serious financial crisis since the Depression, the world watches soaring markets with a mixture of serenity and glee. Natural impulses make finance a neck-snappingly volatile affair. Governments, though, deserve heaps of blame for policies that amplify both boom and bust. As regulators begin picking apart reforms only just enacted, it is worth asking why that is so.

Finance is hopelessly prone to wild cycles. When an economy is purring, profits go up, as do asset values. Rising asset prices flatter borrowers’ creditworthiness. When credit is easier to obtain, spending goes up and the boom intensifies. Eventually perceptions of risk shift, and tales of a “new normal” gain credence: new technologies mean profits can grow for ever, or financial innovation makes credit risk a thing of the past. But when the mood turns, the feedback loop reverses direction. As asset prices fall, banks grow stingier with their loans. Firms feel the pinch from falling sales, get behind on their debts and sack workers, who get behind on theirs. The desperate sell what they can, so asset prices tumble, worsening the crash. Mania turns to panic.

The pattern is an ancient one. In their book “This Time is Different”, Carmen Reinhart and Kenneth Rogoff, two...